Until recently, manufacturers were concerned essentially with the safety of passengers in a vehicle.
Numerous solutions have appeared for that purpose consisting in designing bumpers and in stiffening the external structure thereof so as to form a barrier for protecting the vehicle cabin.
Such bumpers generally comprise a shield, also referred to as a bumper “skin”, acting essentially to provide decoration, but also serving to absorb impacts of small amplitude. Bumpers also comprise one or more beams extending transversely behind the shield and forming a barrier to protect the vehicle cabin. In general, the beam is placed at the same level as the side rails of the vehicle, a height which corresponds substantially to the height of the knee of an adult pedestrian passing in front of the vehicle.
In order to preserve the vehicle cabin and thus its passengers as well as possible, it was considered a few years ago that the structure of the vehicle and thus of the bumper should be as rigid as possible. Consequently, bumpers were used of the kind shown in section in FIG. 1a, in which the shield (100) is placed in the immediate vicinity of the beam (101).
The problem with that type of bumper lies in the structure being too rigid. Progress made in designing structures and in simulating impacts (commonly referred to as “crash testing”), have revealed that a predetermined amount of deformation of the vehicle structure makes it possible to increase safety within the cabin.
Another problem with such bumpers consists in the way the force (F) suffered by a pedestrian as a function of the extent (e) to which the pedestrian is pushed into the shield is at a maximum (F=FMAX) at the moment of impact between the vehicle and the pedestrian, as can be seen from the graph of FIG. 1b. Consequently, the impact suffered by the pedestrian leads to severe lesions of the legs, even if the impact takes place at low speed.
To remedy those problems, manufacturers have turned towards making bumpers comprising materials that are softer in order to absorb impact.
An example of that type of bumper is shown in FIG. 2a, where a bumper shown in section comprises a beam (111) placed behind a shield (110), at a certain distance therefrom. It is possible to make such a bumper because of progress made in injecting plastic materials and in fitting together bodywork parts, thus making it possible for the lines of the shield to be rounded and for it to be placed further away from the beam.
Another way of absorbing shock is shown in FIG. 3a where it can be seen that the bumper comprises a core (122) made of a thermoplastic foam filling the space between the shield (120) and the beam (121) so as to absorb a little more of the impact suffered by the shield.
As can be seen in the graphs of FIGS. 2b and 3b, the problem with such absorption means lies in the way the force suffered by a pedestrian in the event of a head-on collision is at a minimum (F=FMIN) at the moment of impact, and passes instantaneously to a maximum value (F=FMAX) once the shield becomes pressed against the beam.
Thus, in the example of FIG. 3a, even if the variation in the force suffered by the pedestrian during indentation (i.e. the difference between the maximum force FMAX and the minimum force FMIN) is less than that suffered by the pedestrian in earlier situations, the force suffered during impact still remains large.
Consequently, a pedestrian suffering a head-on collision with a bumper of the above-described type will be subjected to a strong force against the leg, and in particular against the knee, and that can lead to severe lesions. These lesions can be the result both of impact proper and of the leg bending excessively about the knee.
Thus although the safety of vehicle passengers has been improved, the problem consists in improving the safety of pedestrians by avoiding lesions at the level of a pedestrian's knee, a zone that is particularly fragile and difficult to treat, and to do so by making impact as non-violent as possible.
In order to preserve the knee joint, recent standards define the maximum angle (about 20°) that the bottom portion of the leg should take up relative to the top portion in the event of an impact.
One solution for complying with these standards consists in using a low restrainer beneath the bumper. A low restrainer arranged at the bottom of the bumper has two effects. Firstly it distributes the force suffered by the leg over two impact zones, and secondly in the event of a collision with a pedestrian, it prevents the bottom of the pedestrian's tibia from passing under the vehicle, thereby restricting the angle to which the leg is bent. Such low restrainers are in widespread use in present-day bumpers.
Nevertheless, the problem with such low restrainers lies in imposing a particular appearance on the bumpers of the vehicle. Such a constraint can degrade the attractiveness of the vehicle.